1. Field of the Invention
The present invention relates to an electric current supply control apparatus for a glow plug which controls the supply of electric current to a heat generation resistor of a glow plug mounted to an engine, and to a glow plug and electric current supply control apparatus connected to the glow plug.
2. Description of the Related Art
Conventionally, a technique has been known called exhaust gas recirculation (hereinafter, also referred to as “EGR”) in which exhaust gas discharged from an engine of an automobile is partially returned to the intake side and reintroduced into the engine. When EGR is performed, the concentration of oxygen within air taken into the engine decreases, whereby combustion of fuel becomes less explosive. In addition, in the intake air, the concentration of carbon dioxide, which has a higher heat capacity than oxygen, increases. Thus, an increase in combustion temperature is suppressed. As a result, the production of nitrogen oxide (NOX) is suppressed, and the concentration of NOX within the exhaust gas can be reduced.
Meanwhile, when an EGR amount is increased, the temperature of the intake air decreases, and the oxygen concentration also decreases. As a result, combustion of fuel becomes unstable, which may cause a reduction in engine output or a misfire. The EGR amount refers to an amount of recirculation of exhaust gas. In some cases, the EGR amount refers to a ratio of the amount of exhaust gas circulated from a circulating passage to the amount of exhaust gas discharged from the engine. However, when the EGR amount is represented by a ratio (%) in relation to the present invention, it represents a ratio of the amount of circulated exhaust gas (hereinafter referred to as EGR gas) to the amount of a mixture of atmospheric air and EGR gas introduced into a cylinder of the engine. This ratio can be obtained from CO2 concentration through calculation {(CO2 within EGR gas within intake gas)−(CO2 within atmospheric air)}/{(CO2 within exhaust gas)−(CO2 within atmospheric air)}. FIG. 6 shows a specific example of the above-mentioned phenomenon. In a test for investigating such a phenomenon, an engine for testing was used, and the engine speed was sharply increased from an idling state (about 700 rpm) with an accelerator pedal fully depressed. The solid line shows the increase in engine speed as a function of time for the case where EGR was not performed (EGR amount: 0%). Further, the single-dot chain line shows the increase in engine speed for the case where the EGR amount was set to 10% under the same conditions; and the two-dot chain line shows the increase in engine speed for the case where the EGR amount was set to 14.9%. Further, the broken line A shows the increase in engine speed for the case where the EGR amount was set to 18.8%, and the broken line B shows the increase in the engine speed for the case where the EGR amount was set to 23%. The results of this test also reveal that, irrespective of the EGR amount, the engine speed became saturated at about 3000 rpm, and no difference was observed in terms of the maximum speed of the engine. However, the results of this test reveal that, the greater the EGR amount, the longer the time required by the engine to increase its speed from about 700 rpm to about 3000 rpm. That is, the results of this test show that, when the EGR amount is increased, torque decreases.
Incidentally, when an engine has not yet warmed sufficiently, such as at the time of cold startup of the engine or immediately after startup, a glow plug is used to assist in the startup or to allow for stable operation. In such a case, since the intake air temperature is low, a reduction in NOX can be expected. As is known, the temperature of a glow plug is desirably set to 1100° C. or higher in order to reduce NOX while securing ignition performance (see, for example, Patent Document 1). Further, although the EGR amount desirably is increased so as to further reduce NOX, the increased EGR amount makes combustion unstable. In order to overcome such a drawback, a technique has been proposed in which, at the time of cold start of an engine when electric current is supplied to a glow plug, the EGR amount is increased so as to more reliably suppress the generation of NOX (for example, see Patent Document 2).
The present inventors found that, not only when an engine is cold-started but also when the load acting on the engine increases at the time of acceleration or the like, the engine can be operated stably by supplying electric current to a glow plug. FIG. 7 specifically shows the results of a test. In this test, the above same engine for testing was used, and the engine was accelerated from an idling state (about 700 rpm) with the accelerator pedal fully depressed. The solid line shows an increase in engine speed as a function of time for the case where EGR was not performed (EGR amount: 0%), and this increase in engine speed was used as a reference. The glow plug was heated to 1100° C., and the test was performed while the EGR amount was adequately changed. The changed EGR amount was recorded when an increase in engine speed as a function of time approximately the same as the reference was attained (i.e., when the time required to increase the engine speed from about 700 rpm to about 3000 rpm and the maximum engine speed became approximately the same as those in the case of the reference) as indicated by a single-dot chain. The recorded EGR amount was 7.2%. Similarly, the test was performed while the glow plug was heated to 1200° C. and 1300° C., and the changed EGR amount was recorded when an increase in engine speed as a function of time approximately the same as the reference was attained (indicated by a two-dot chain line and a broken line, respectively). The recorded EGR amounts were 8.9% and 10.9%, respectively. The results of this test reveal that, even in a situation where the load imposed on the engine increases such as at the time of acceleration, if the glow plug is heated to thereby increase its temperature, combustion can be made more stable, whereby the EGR amount can be increased without reducing the output of the engine.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2008-89233    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2008-196311
3. Problems to be Solved by the Invention
When the accelerator pedal of an automobile is depressed, since the amount of injected fuel increases, a rich atmosphere tends to be created, and an increase or decrease (change) in the NOX generation amount may become large. In order to reduce NOX, preferably, the EGR amount is increased as much as possible. However, when the speed of an engine increases, a glow plug attached to the engine becomes susceptible to an influence of swirl produced within the engine or the like, and its temperature is apt to change and become unstable. Specifically, when the load imposed on the engine increases sharply as shown in FIG. 8 (when the engine speed increases within a short period of time), the temperature of the glow plug also changes greatly. In this example, the temperature of the glow plug changes within a range of about 220° C. When the temperature of the glow plug temporarily decreases because of influence of swirl or the like, torque may be reduced and drivability may be impaired unless the EGR amount is proper. In order to optimize the EGR amount, the temperature of the glow plug at the time of acceleration is desirably stabilized. However, the heat generation state of the glow plug and the state of generation of swirl within the engine vary among automobiles. Accordingly, unless the relationship between the temperature of the glow plug and increase in engine speed as a function of time is determined for each engine through mapping, stabilization of the temperature of the glow plug is difficult. In addition, mapping increases cost. Therefore, conventionally, priority is given to securing drivability, and EGR, which may cause a reduction in torque, is not performed at the time of acceleration.
The present invention has been accomplished in order to solve the above-described problems, and an object of the present invention is to provide an electric current supply control apparatus for a glow plug which can supply electric current to the glow plug when a load imposed on an engine is in a transition state, to thereby enable the circulating amount of exhaust gas to increase. Another object of the present invention is to provide a glow plug which is connected to the electric current supply control apparatus and whose energization state is thereby controlled.